www.clim-past.net/12/429/2016/ doi:10.5194/cp-12-429-2016
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Late Cretaceous (late Campanian–Maastrichtian) sea-surface
temperature record of the Boreal Chalk Sea
Nicolas Thibault1, Rikke Harlou1, Niels H. Schovsbo2, Lars Stemmerik3, and Finn Surlyk1
1Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10,
1350 Copenhagen, Denmark
2Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen, Denmark
3Statens Naturhistoriske Museum, University of Copenhagen, Øster Voldgade 5–7, 1350 Copenhagen, Denmark
Correspondence to:Nicolas Thibault (nt@ign.ku.dk)
Received: 26 September 2015 – Published in Clim. Past Discuss.: 3 November 2015 Revised: 29 January 2016 – Accepted: 9 February 2016 – Published: 24 February 2016
Abstract.The last 8 Myr of the Cretaceous greenhouse inter-val were characterized by a progressive global cooling with superimposed cool/warm fluctuations. The mechanisms re-sponsible for these climatic fluctuations remain a source of debate that can only be resolved through multi-disciplinary studies and better time constraints. For the first time, we present a record of very high-resolution (ca. 4.5 kyr) sea-surface temperature (SST) changes from the Boreal epicon-tinental Chalk Sea (Stevns-1 core, Denmark), tied to an as-tronomical timescale of the late Campanian–Maastrichtian (74 to 66 Ma). Well-preserved bulk stable isotope trends and calcareous nannofossil palaeoecological patterns from the fully cored Stevns-1 borehole show marked changes in SSTs. These variations correlate with deep-water records of climate change from the tropical South Atlantic and Pacific oceans but differ greatly from the climate variations of the North At-lantic. We demonstrate that the onset and end of the early Maastrichtian cooling and of the large negative Campanian– Maastrichtian boundary carbon isotope excursion are coin-cident in the Chalk Sea. The direct link between SSTs and δ13C variations in the Chalk Sea reassesses long-term glacio-eustasy as the potential driver of carbon isotope and climatic variations in the Maastrichtian.
1 Introduction
Superimposed on the long-term cooling trend of the lat-est Cretaceous, two benthic foraminiferal positive oxygen isotope excursions have been documented in the early and late Maastrichtian at low and mid-latitudes of the North and
South Atlantic, Indian Ocean, and central Pacific (Barrera and Savin, 1999; Friedrich et al., 2009). These positive excur-sions, which likely reflect bottom-water cooling, have been tentatively correlated to third-order sea-level falls and associ-ated with changes in the mode and direction of thermohaline oceanic circulation, possibly caused by the build-up of small ephemeral Antarctic ice sheets (Barrera and Savin, 1999; Miller et al., 1999). Alternatively, these climatic changes have been associated with shifts in the source of deep wa-ter formation from low to southern high latitudes, linked to the opening of deep-sea gateways in the South Atlantic (Friedrich et al., 2009; Robinson et al., 2010; Moiroud et al., 2016). In addition, the latest Maastrichtian was char-acterized worldwide by a brief greenhouse warming pulse, linked to Deccan volcanism (Li and Keller, 1998a; Robin-son et al., 2009). This latter event is well-recorded in oxygen isotopes of benthic foraminifera but is poorly expressed in their planktonic counterparts (Li and Keller, 1998a, b; Bar-rera and Savin, 1999; Abramovich et al., 2003). Neverthe-less, changes in the marine plankton community at the end of the Maastrichtian suggest a drastic, but as of yet poorly con-strained, increase in global sea-surface temperatures (SSTs, Abramovich et al., 2003; Thibault and Gardin, 2010).
glob-430 N. Thibault et al.: Late Cretaceous (late Campanian–Maastrichtian) sea-surface temperature record
Figure 1.(a) Palaeogeographic reconstruction for the Maastrichtian (66 Ma) showing location of the Boreal Chalk Sea (square) and key localities discussed in the text (after Markwick and Valdes, 2004, modified). (b) Palaeogeographic reconstruction of the Boreal Chalk Sea for the Maastrichtian with location of Stevns-1 (after Surlyk et al., 2003, modified).
ally cooling in all other oceanic basins (MacLeod et al., 2005). These regional differences emphasise the need for well-calibrated high-resolution data from different basins, and from open ocean and epicontinental seas, in order to pro-vide a reliable picture of past climates. Data from the mid-latitude Boreal epicontinental Chalk Sea are particularly crit-ical as this basin was connected to the North Atlantic Ocean to the west, to the Tethys to the southeast and possibly to the Arctic Ocean to the north (Fig. 1).
To investigate climate change in the Boreal Chalk Sea, we generated a calcareous nannofossil temperature index (NTI) and a new record of 1932 bulk carbonate stable isotopes across the late Campanian–Maastrichtian of the Stevns-1 core, Denmark (see Supplement). The sedimentology and stratigraphy of Stevns-1 have been described in detail by Rasmussen and Surlyk (2012) and Surlyk et al. (2013). Car-bon isotope stratigraphic correlations with ODP Site 762C have been used to tie the Stevns-1 record to the astronom-ical timescale of the late Campanian–Maastrichtian (66 to 74.5 Ma, Fig. 2).
2 Methods
2.1 Age model
The age model is based on the correlation of magnetostrati-graphic records at sites 762C and 525A and carbon iso-tope curves of Stevns-1, 762C and 525A as presented in
Figure 2.Age model for the Stevns-1 core based on the correla-tion of carbon-isotope curves of Stevns-1 with the astronomically calibrated ODP Site 762C and DSDP Site 525A.
Figure 3.Calcareous nannofossil climatic data and bulk stable isotopes of Stevns-1. Background colours delineate cool and warm climatic trends in the Chalk Sea. (1) Thibault et al. (2012b). (2) Surlyk et al. (2013) for details of the dinoflagellate biozonation and lithostratigraphy. (3) Rasmussen and Surlyk (2012) for the full sedimentological description of the Stevns-1 core. (4) Age model after Thibault et al. (2012a) and correlation of Fig. 2.
2.2 Isotopic measurements and palaeotemperature reconstruction
Oxygen and carbon isotopic ratios of bulk carbonates were measured on a micromass isoprime spectrometer. Analyti-cal precision is Analyti-calculated as 0.1 for δ18O and 0.05 ‰ for δ13C. Sea-surface temperature estimates (Fig. 3) are based on Anderson and Arthur (1983) for bulk carbonates of Stevns-1 and equation (Stevns-1) of Bemis et al. (Stevns-1998) for foraminiferal data of Site 525A, using aδ18Osw of Late Cretaceous sea-water of −1.0 ‰ SMOW for an ice-free world. Resulting average SST estimates of ca. 15.5◦C in the early
Maas-trichtian of Denmark are in agreement with the global compi-lation of Zakharov et al. (2006). In addition, we provide tem-perature estimates for the bulk carbonate of Stevns-1 using an average δ18Oswof Late Cretaceous seawater of−0.5 ‰ SMOW (Fig. 3) assuming glacio-eustatic variations in the range of 25–75 m in the Maastrichtian, by comparison with the extent of coincident δ18Osw and sea-level variations in the Oligocene to Early Miocene (Billups and Schrag, 2002).
2.3 Calcareous nannofossil data
A total of 89 nannofossil slides were prepared following the method described in Thibault and Gardin (2006). Slides were
analysed for quantitative counts. Preservation of the assem-blage is moderate in all samples. Relative abundances have been calculated for a total of more than 400 specimens. Our nannofossil temperature index (NTI) was calculated as the ratio between warm-water taxa and the sum of warm-water and cool-water taxa identified in the assemblage (see Sect. 3).
3 Results and interpretations
3.1 Calcareous nannofossils
vary-432 N. Thibault et al.: Late Cretaceous (late Campanian–Maastrichtian) sea-surface temperature record
ing abundances and patterns of migration of this species in mid-latitude and high latitudes have mainly been linked to temperature changes (Watkins, 1992; Sheldon et al., 2010). Sheldon et al. (2010) and Thibault et al. (2015) have shown that warm intervals in the Boreal Chalk Sea are character-ized by an increase in abundance ofW. barnesiae. Cool-water taxa andW. barnesiaeclearly show opposite long-term trends throughout the studied succession and highlight three warm intervals in the late Campanian, mid-Maastrichtian and lat-est Maastrichtian, and two cool intervals in the early and late Maastrichtian (Fig. 3). The palaeoecological affinity of Ah-muellerella regularisis unknown and this species is known to be common both in the tropical and mid-latitude areas. How-ever, in the Stevns-1 core, intervals with lower abundances of high-latitude taxa and higher abundances inW. barnesiae
also show enrichments inA. regularis(Fig. 3). Therefore, in this study, this species was grouped together withW. barne-siaeamong the warm-water taxa. Our nannofossil tempera-ture index (NTI) was calculated as follows:
NTI = (%W. barnesiae+%A. regularis)/
(%A. cymbiformiss.l.+%A. octoradiata
+%Gartneragospp.+%K. magnificus
+ %N. frequens)+(%W. barnesiae
+ %A. regularis)
. (1)
Note that the applicability of the NTI developed here is re-stricted to the Campanian–Maastrichtian interval and should preferably remain valid for the Boreal Realm only as the composition of the calcareous nannofossil assemblage is sig-nificantly different in the tropical realm.
3.2 Validation of Stevns-1 bulkδ18O as a proxy for Late
Cretaceous SSTs
The nannofossil chalk of the Danish Basin is a very pure car-bonate. Carbonate content of the analysed bulk rock sam-ples is generally over 95 % in all analysed samsam-ples. Scanning electron microscope images of the chalk show numerous cal-careous nannofossils with dissolution phenomena, little re-crystallisation and small carbonate micro-particles, which likely come from the breakdown of nannofossils (Fig. 4). De-spite its controversial use, a number of studies have demon-strated the usefulness of bulk pelagic/hemipelagic carbonate oxygen stable isotope data for SST reconstruction (Jarvis et al., 2011, 2015; Reghellin et al., 2015). Recent experi-ments have shown reduced coccolithophore interspecific dif-ferences and less carbon and oxygen-isotope fractionation in large and slow-growing species with higher ambient carbon availability, which is expected in the sea water of periods with high CO2concentrations such as the Cretaceous (Rick-aby et al., 2010; Bolton et al., 2012; Hermoso et al., 2014). It has been recently proven that coccolithophore vital effects actually vanish at highpCO2 regimes, thus supporting the
Figure 4.SEM picture of the Stevns-1 chalk. Sample 6146 (late Maastrichtian cooling episode, nannofossil subzone UC20b-cBP, depth: 73.91 m). Two of the main cool-water nannofossil taxa are shown. Bar is 10 µm.
use of non-altered bulk nannofossil chalk as an excellent cal-citic material forδ18O analysis as a proxy for SSTs, despite recent years of neglect and favour to species-specific planktic foraminiferδ18O (Hermoso et al., 2016). Due to its limited diagenetic alteration, the nannofossil chalk of Stevns-1 may thus draw a reliable picture of sea-surface water environmen-tal conditions.
The NTI andδ18O values of Stevns-1 show similar trends for the late Campanian–Maastrichtian (Fig. 3). In order to test the correlation between low-resolution nannofossil and high-resolution isotopic data, both 7- and 21-point running averages were run overδ18O values. Correlation was tested for the resulting values of the 89 samples in common. The R2Pearson coefficient of correlation between the NTI and the δ18O is 0.60 for the 7-point running average and in-creases up to 0.69 for a 21-point running average. This suggests temperature (and possibly continental ice growth driven changes in seawater δ18O) as the dominant(s) fac-tor(s) controlling the observed variations in the bulkδ18O of Stevns-1. Little influence of diagenesis is reinforced by the lack of correlation betweenδ18O andδ13C data of this core (Thibault et al., 2012b). The timing and magnitude of long-termδ18O changes at Stevns-1 match those recorded in ben-thic foraminiferal data at Site 525A (Li and Keller, 1998a, b) as well asδ18O variations observed in planktonic foraminifer
Figure 5.Stable oxygen isotope and calcareous nannofossil data of Stevns-1 compared to data on foraminifers of South Atlantic DSDP Site 525A. The age scale is based on the correlation of carbon isotope curves between Stevns-1, DSDP Site 525A and the astronomically calibrated ODP Site 762C. La2010d: astronomical solution from Laskar et al. (2011). Benthic and planktonic foraminiferal stable isotope data from Li and Keller (1998a, b) and Friedrich et al. (2009).
3.3 Climatic trends
Calcareous nannofossil and δ18O data from Stevns-1 sug-gest the following climatic evolution in the Chalk Sea: from 73.8 and 72.8 Ma, a late Campanian climatic warm opti-mum with SSTs between ca. 19 and 20◦C, followed by a
late Campanian–early Maastrichtian progressive cooling of 3.5◦
C (Fig. 5). From 72.8 to 71 Ma, cooling occurs in two major phases, interrupted by a ca. 600 kyr long stable period. These two long cooling steps are characterized by rapid tem-perature drops, two of which can also be observed in the NTI at 72.8 and at 72–71.8 Ma. A cool climate mode with slight oscillations in SSTs around 15.5◦C is recorded between 71
and 69.8 Ma. The transition between the early Maastrichtian cooling and mid-Maastrichtian warming is characterized by
a rapid 1.5◦
C increase in SSTs lasting ca. 300 kyr at 69.8– 69.5 Ma. The mid-Maastrichtian warming trend is character-ized by SSTs oscillating around 16.5 to 17◦C between 69.5
and 68.4 Ma. This trend is followed by a late Maastrichtian cooling of 1◦C during which minimal SSTs around 15.5◦C
at 67.9 Ma are reached. A concomitant sharp decrease in δ18O and increase of the NTI delineate the end-Maastrichtian warming from ca. 66.3 Ma up to the K–Pg boundary. This last warming episode accounts for a total of 2◦C in the
Bo-real Realm and occurs in two rapid (< 100 kyr) steps of 1◦C
each at ca. 66.3 and 66.2 Ma (Fig. 5).
434 N. Thibault et al.: Late Cretaceous (late Campanian–Maastrichtian) sea-surface temperature record
developed across the K–Pg boundary. Sections through these basins show a complete development of the K–Pg succes-sion, whereas sections between the basins contain a small gap spanning the boundary transition and in the order of sev-eral tens of centimetres, i.e. < 30 kyr (Surlyk et al., 2006). In addition, the presence of a double incipient hardground at the base of the Højerup Member, situated 4 to 5 m below the K–Pg boundary at Stevns Klint, suggests another potential hiatus within the uppermost Maastrichtian interval (Surlyk et al., 2006). These small gaps in Stevns-1 partly hinder obser-vation of a globally characterized short cooling event imme-diately before the K–Pg event (Li and Keller, 1998a; Thibault and Gardin, 2010; Punekar et al., 2014; Thibault and Husson, 2015). The duration of this end-Cretaceous cooling episode has been estimated to be of the order of 100 to 120 kyr and is followed by a ca. 30 kyr short pulse of warming in the Tethys (Punekar et al., 2014; Thibault and Husson, 2015).
A sharp decrease in the relative abundance ofW. barnesiae
and in the NTI situated within the Højerup Member suggests that the onset of the last end-Cretaceous cooling episode is present in the core (Fig. 3). Therefore, the cumulated upper-most Maastrichtian gap in the Stevns-1 core is of a limited extent, corresponding to less than 150 kyr of the latest Cre-taceous and most of it probably corresponds to the double incipient hardground.
4 Discussion
Despite regional differences between the South Atlantic and the Chalk Sea, climatic trends generally match well (Fig. 5). Except for the North Atlantic, low and mid-latitudes of all other oceanic basins show the same climate modes (Barrera and Savin, 1999; MacLeod et al., 2005) that appear to be related to coeval changes in atmospheric pCO2 (Nordt et al., 2003; Gao et al., 2015). Differences in climate trends between the Chalk Sea and the North Atlantic are difficult to reconcile with their tight connection and multiple gate-ways across submerged parts of UK and the Paris Basin (Fig. 1). However, Maastrichtian water masses at the location of Stevns-1 could have been influenced by Tethyan north-westward currents, as suggested by the direction of large channels in seismic profiles offshore Stevns Klint and fur-ther north in the Danish Basin (Lykke-Andersen and Surlyk, 2004; Esmerode et al., 2007; Surlyk and Lykke-Andersen, 2007).
Climatic trends from Stevns-1 correlate withδ18O trends of benthic foraminifers at Site 525A (Walvis Ridge, South Atlantic), which represents the highest-resolution record on separated foraminifers for this time interval (Li and Keller, 1998a, b; Friedrich et al., 2009) (Fig. 5). SSTs in the Chalk Sea followed the same evolution as intermediate and deep waters of the South Atlantic, Indian Ocean and central Pa-cific (Barrera and Savin, 1999). However, the record of most planktonic foraminifers in these basins fails to clearly
de-pict the trends in sea-surface waters (Barrera and Savin, 1999; Li and Keller, 1999). Stevns-1 bulk δ18O and cal-careous nannofossil assemblages, as well asδ18O values of
G. arcaat Site 525A, indicate that climatic modes recorded in late Campanian–Maastrichtian intermediate and deep wa-ters affected sea-surface wawa-ters in a similar fashion (Fig. 5). Changes in Maastrichtian nannofossil assemblages in the tropical Atlantic and Pacific oceans delineate the same cli-matic trends in sea-surface waters (Thibault and Gardin, 2006, 2010). Migration patterns in planktonic organisms throughout the late Campanian–Maastrichtian strengthen this interpretation (Watkins, 1992; Thibault et al., 2010).
When comparingδ18O values of planktonic foraminifers
Rugoglobigerina rugosa and Globotruncana arca at Site 525A across the early Maastrichtian cooling, it appears that data from the latter species show a better consistency with variations observed in the benthic foraminiferal record and in the bulk of Stevns-1 (Fig. 5). This suggests thatδ18O val-ues ofR. rugosalikely reflect underestimated and smoothed SST variations in the late Campanian–Maastrichtian inter-val, whereas data acquired onG. arca reflect a more faith-ful picture of primary Late Cretaceous South Atlantic SSTs. Potential diagenetic overprint, the presence of numerous pseudo-cryptic ecophenotypes of Rugoglobigerina and vi-tal effect associated with photosymbiosis can be invoked to explain this discrepancy in the δ18O values of R. rugosa
(Abramovich et al., 2003; Falzoni et al., 2014).
Despite these similarities, regional differences are never-theless apparent across the early Maastrichtian cooling event between Stevns-1 (palaeolatitude: 45◦N) and Site 525A
(palaeolatitude: 36◦S). A short warming pulse occurs in the
middle of this interval at Site 525A (Fig. 5). This pulse is very poorly recorded at Stevns-1 through a number of outlying data points with lighter values between −252 and −272 m (Fig. 3) and through slightly lighter values in the 2 standard deviation envelope of Stevns-1 between 70.45 and 70.85 Ma (Fig. 5). The nannofossil assemblage of Stevns-1 does not appear to show any warming in this interval. However, a similar short pulse of warming has been recorded during the early Maastrichtian cooling through lighter values of the bulk δ18O accompanied by a slight enrichment in Watznaueria barnesiaein the Skælskør-1 core (SW Sjælland, Denmark; Thibault et al., 2015).
actu-ally supports synchroneity between the CIE and the positive δ18O excursion of this interval. Isotopic data from Site 525A neither confirm nor refute this observation because data from the onset of the excursion at 73 Ma are lacking. However, the return to a mid-Maastrichtian warm mode at 69.5 Ma is coincident with a rapid increase in theδ13C of benthic and planktonic foraminifers at Site 525A (Fig. 2). Therefore, it is possible that the lag between the two signals is a South-ern Ocean phenomenon. Decoupling between the two sig-nals can, however, be highlighted at Stevns-1 elsewhere in the record. For example, the stepwise decrease in δ13C be-tween 73 and 71 Ma appears to be decoupled from theδ18O record (Fig. 3). Here, maximum cooling occurs between 71.5 and 69.5 Ma during a progressive rise inδ13C values. This decoupling remains to be explained, but with respect to the onset and termination ofδ18O andδ13C excursions, our re-sults argue for a direct cause-and-effect scenario in the Chalk Sea.
Decoupling and lead–lag relationships between these two proxies have been used as an argument to rule out Maas-trichtian glaciation and a subsequent drop in sea level as a likely scenario for these two isotopic excursions (Friedrich et al., 2009). On the contrary, our results tend to show consis-tency with a glacio-eustatic scenario as previously supported by Barrera and Savin (1999) and Miller et al. (1999). Despite the fact that Kominz et al. (2008) usedA Geologic Time Scale 2004 with the K–Pg and Campanian–Maastrichtian bound-aries at 65.5 and 70.6 Ma, respectively, comparison of the timing of the two cooling episodes appears to match fairly well with that of the two major lowstands in the New Jer-sey margin sea-level curve (Kominz et al., 2008; Fig. 5). Haq (2014) recently identified six third-order sea-level cy-cles in the late Campanian–Maastrichtian interval bounded by sequence boundaries (SBs) KCa7, KMa1, KMa2, KMa3, KMa4 and KMa5, among which KMa2 and KMa5 at 70.6 and 66.8 Ma, respectively, are considered as major cycle boundaries. Considering the great uncertainty in the esti-mated ages of Haq’s SBs, the timing of major SBs KMa2 and KMa5 at 70.6 and 66.8 Ma corresponds well to a position within the two lowstands of the New Jersey record and within the cooling episodes highlighted at Stevns-1 (Fig. 5). A num-ber of third-order sea-level regressions of the mid-Cretaceous greenhouse have been recently explained through aquifer-eustasy (Wagreich et al., 2014; Wendler and Wendler, 2016; Wendler et al., 2016). However, such regressions correlate to climatic warming episodes and this new model for sea-level change can thus not explain the relationship between global cooling and third-order sea-level fall mentioned here for the Maastrichtian (Wendler and Wendler, 2016; Wendler et al., 2016).
Although no direct evidence of glaciation, such as drop-stones and ice-rafted debris, has been found in the late Campanian–Maastrichtian of the Southern Ocean (Price et al., 1999), examination of diatom-rich sediments from the Alpha Ridge, and palynomorph records from southeastern
Australia and Seymour Island support the development of winter sea ice in the Arctic Sea and around Antarctica, and the waxing and waning of ephemeral Antarctic ice sheets at that time (Gallagher et al., 2008; Davies et al., 2009; Bow-man et al., 2013). The development of ephemeral ice sheets in Antarctica can explain theδ18O excursions through a drop in seawaterδ18O accompanied by a global cooling of water masses (Barrera and Savin, 1999; Li and Keller, 1999). Sea-level changes could trigger the onset and termination of the late Campanian–early Maastrichtian CIE by shifting calcium carbonate accumulation and organic-matter burial from shelf to open-ocean areas (Barrera and Savin, 1999; Friedrich et al., 2009). The occurrence of the CIE and the early Maas-trichtian cooling have been recently explained mainly by a global change in the source of intermediate and deep-water masses and the onset of deep-water formation in the Southern Ocean, favoured by the opening of tectonic gateways (Robin-son et al., 2010; Koch and Friedrich, 2012). However, a reor-ganisation in the global oceanic circulation is actually com-patible with a glacio-eustatic scenario and could have been triggered both by tectonics and glaciation. In such a scenario, changes in the seawaterδ18Oswwithin a range of 25 to 75 m glacio-eustatic variations may have followed a rather similar evolution as for the Oligocene–Miocene interval (Billups and Schrag, 2002). Palaeotemperature calculations should pro-gressively and cyclically shift from an equation that assumes a seawaterδ18O (δ18Osw) of−1 ‰ to potentialδ18Oswdown to ca.−0.5 ‰ (Billups and Schrag, 2002). Minimum tem-peratures of 15.5◦C for the SSTs of the Boreal Chalk Sea
during the early and late Maastrichtian coolings could thus be underestimated and may rather be around 17.5◦C (Figs. 3
and 5).
5 Conclusions
equa-436 N. Thibault et al.: Late Cretaceous (late Campanian–Maastrichtian) sea-surface temperature record
tions using either aδ18Oswof−1 ‰ during warm episodes or aδ18Oswof ca.−0.5 ‰ during cool episodes. In such a sce-nario, the full extent of the early Maastrichtian SST cooling would thus be 4◦
C rather than 6◦
C. Finally, the two sharp stepwise 1◦C increases in SSTs of the Chalk Sea at 66.3
and 66.2 Ma are consistent with the second main phase of the Deccan volcanic episode in a series of rapid pulses of flood basalt volcanism and associated release of greenhouse gases (Chenet et al., 2009).
The Supplement related to this article is available online at doi:10.5194/cp-12-429-2016-supplement.
Acknowledgements. Funding for this study was provided by the Danish Natural Science Research Council, the Natural Science Faculty, the University of Copenhagen and the Carlsberg Foundation. We thank C. V. Ullmann, J. Moreau, M. Ruhl and D. Husson for fruitful discussions, as well as M. Wagreich and I. Jarvis for their useful comments and suggestions on the article.
Edited by: A. Haywood
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